Comparative Transcriptomic Analysis of Zymoseptoria tritici Reveals Interaction-Specific Gene Expression Patterns During Susceptible, Resistant, and Non-Host Interactions

Zymoseptoria tritici causes Septoria tritici blotch which significantly reduces yields of wheat. To investigate infection phase-specific gene expression in the pathogen, we analyzed gene expression during infection of susceptible and resistant wheat cultivars, plus the non-host species barley at 1, 3, 6, 10, 17 and 23 days post inoculation (DPI). There were dramatic dif-ferences in pathogen gene expression at 10 DPI in the susceptible compared to both re-sistant interactions. The most pronounced differences in pathogen gene expression were observed at 3 DPI in both the susceptible and resistant host interactions compared to the non-host. Thirty-one putative effectors showed early expression during the susceptible compared to the non-host interaction; six were selected for subcellular localization studies. Using Agrobacterium-mediated transient expression in Nicotiana benthamiana, subcellular localization assays revealed that two candidate effectors localized to putative mobile cyto-solic bodies when expressed without their signal peptides, suggesting potential roles in in-tracellular signaling or host gene regulation. When expressed with their signal peptides, four candidate effectors localized to the cytosol, while one did not accumulate to detectable levels. Comparison of pathogen gene expression in the susceptible host to expression in the resistant hosts identified genes expressed during the transition from biotrophic to necrot-rophic growth at 10 DPI. Comparison of pathogen gene expression in resistant and suscep-tible hosts, versus in the non-host barley, identified genes involved in initial colonization and host recognition. These results contribute to understanding candidate effectors that are activated early during infection and may play a role in the suppression of plant immunity.